Wave form measurement at internal nodes in integrated circuits is necessary both to verify the circuit's design and to analyze its failures. Numerous systems are known which perform such measurements, the most common systems using either electron probes or mechanical probes to access the internal nodes.
Prior art electron probes, most of which are essentially scanning electron microscopes, have several problems which limit their usefulness as debugging tools. First, the necessity of maintaining the integrated circuit being tested in a vacuum makes it difficult to access the internal nodes with a mechanical probe to inject test signals. Second, there is no simple way for the tester to locate the specific area on the integrated circuit where test signals must be injected or received. Locating a specific area on the integrated circuit requires extensive physical manipulation of the circuit or viewing apparatus until the area of the circuit being viewed on the apparatus corresponds to the area of the suspected malfunction as that area is drawn on the schematic diagram of the circuit being tested. This process can be very time consuming when a plurality of points must be tested. Although Richardson, U.S. Pat. 4,706,019, discloses an electgron beam test probe system wherein the electron probe is closely integrated with a schematic diagram of the integrated circuit, the Richardson System is quite complex. Last, an electron probe cannot inject test signals.
Known mechanical probe systems correct some of these problems but share some of the same problems and add new problems. Mechanical probes can make very accurate voltage measurements and have a frequency response comparable to that of an electron probe. However, as in current electron probe systems, it is difficult to locate the area or component to be tested as no simple correspondence between the schematic which describes the circuit and the area of the actual circuit being viewed exists. Probe systems can also damage the integrated circuit when they are applied to test areas if the probe is overdriven into the circuit. This problem necessitates extreme positioning accuracy for the probe, which greatly increases testing time.
Given the shortcomings of existing electron probes and mechanical microprobes, a need exists for an integrated circuit testing system which can provide rapid access to and testing of specific areas in an integrated circuit, which will be extremely accurate in its voltage measurements, which can inject test signals, but which will not damage the circuit being tested.